Stockholders' Equity - Effect of VE Territory Covered Losses Through Coversion Rate Adjustments (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Conversion of Stock [Line Items]
Recovery through conversion rate adjustment	$ 5	$ 0	$ 5
UK&I preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock (in shares)			0	0
Effective price per share (in dollars per share)			$ 180.00	$ 141.32
Recovery through conversion rate adjustment			$ 72	$ 6
Europe preferred stock
Conversion of Stock [Line Items]
Reduction in equivalent number of as-converted shares of class A common stock (in shares)			1	0
Effective price per share (in dollars per share)			$ 180.00	$ 150.26
Recovery through conversion rate adjustment			$ 92	$ 2